The present invention relates to thermosetting powder compositions comprising as binder a mixture of an amorphous polyester containing carboxyl groups, of a semicrystalline polyester containing carboxyl groups and of a crosslinking agent having functional groups capable of reacting with the carboxyl groups of these polyesters, and in particular to thermosetting powder compositions which, by curing, give coatings which have very good weatherability and have good mechanical properties.
The invention also relates to the use of these compositions for the preparation of powder paints and varnishes allowing these coatings to be produced, as well as to the coatings thus obtained.
Thermosetting powder compositions are well known in the art and are widely used a paints and varnishes for the coating of the most varied of articles. These powders have many advantages; on the one hand, the problem of solvents is completely eliminated, and on the other hand the powders are 100% used, given that only the powder in direct contact with the substrate is retained by the latter, any excess powder being, in principle, completely recoverable and reusable. This is why these powder compositions are preferred to coating compositions in the form of solutions in an organic solvent.
Thermosetting powder compositions have already been widely used in the coating of domestic electrical appliances, accessories for the motor-vehicle industry etc. They generally contain thermosetting organic compounds constituting the binder of the pain, fillers, pigments, catalysts and various additives in order to match their properties to their use.
Various types of thermosetting powder compositions exist. The most widely known compositions contain as binder either a mixture of polymers containing carboxyl groups, such as a carboxyl-group containing polyester or a carboxyl-group-containing polyacrylate, and of epoxy compounds such as triglycidyl isocyanutate or acrylic copolymers containing glycidyl groups or xcex2-hydroxyalkylamides, or a mixture of polymers containing hydroxyl groups, usually a hydroxyl-group-containing polyester, with blocked or unblocked isocyanates, glycolouril or melamine resins, polycarboxylic acid anhydrides, etc.
Polyesters containing carboxyl groups or hydroxyl groups which can be used for the preparation of powder varnishes and paints have already formed the subject of numerous publications. These polyesters are usually prepared from aromatic polycarboxylic acids, mainly terephthalic acid and isophthalic acid and optionally a minor proportion of aliphatic or cycloaliphatic dicarboxylic acids, and from various aliphatic polyols such as, for example, ethylene glycol, neopentyl glycol, 1,4-butanediol, trimethylolpropane, etc. These polyesters based on aromatic dicarboxylic acids, when they are used with a suitable crosslinking agent, provide thermosetting compositions which give paint and varnish coatings having good properties both with regard to their appearance and with regard to their mechanical properties (impact strength, flexibility, etc.). Some of these polyesters and the powders prepared from them are also commonly employed because of their outstanding weatherability properties.
This latter type of polyester generally contains, as acid, mostly isophthalic acid. However, although the coatings obtained from these polyesters have good weatherability, they do not on the other hand have good mechanical strength.
Currently, the great majority of polyesters used in the thermosetting powder compositions are amorphous polyesters. However, when the polyester is amorphous it is difficult to prepare perfect pulverulent thermosetting compositions, because they must satisfy often contradictory criteria. Thus, these powders must not reagglomerate while they are being handled, transported and stored, which means that the amorphous polyester must have a sufficiently high glass transition temperature (Tg). On the other hand, for the powder particles to be able to coalesce and form a perfectly homogenous and uniform coating, it is necessary for its viscosity at the curing temperature, and therefore the glass transition temperature (Tg) of the polyester, to be sufficiently low to ensure good wetting of the pigments and other solid materials accompanying the polyester in the formulation of the said thermosetting powder compositions. In addition, the powder must be capable of melting at the curing temperature in order to form an even film before the crosslinking reaction leading to final curing commences. In order to obtain good spreading of the molten film on the surface of the substrate, it is therefore necessary for the melt viscosity of the polyester to be sufficiently low. This is because a very high melt viscosity prevents good spreading of the molten film and results in a loss of evenness and of gloss in the coating. Finally, the rate of the crosslinking reaction of the composition, at a given temperature, may be controlled only by varying the quantity and/or nature of the crosslinking agent and of the optionally used crosslinking catalyst.
These various problems are solved in powder compositions of which the binder is formed by a semicrystalline polyester containing carboxyl groups or hydroxyl groups, this semicrystalline polyester being used alone or as a mixture with an amorphous polyester containing carboxyl or hydroxyl groups. This is because the properties of semicrystalline polyesters make it possible to avoid, to a large extent, the drawbacks described previously which arise from controlling the glass transition temperature (Tg), the melt viscosity and the reactivity of amorphous polyesters.
Firstly, semicrystalline polyesters have a high melting point and, as the case may be, a low glass transition temperature (Tg). Consequently, the melt viscosity of semicrystalline polyesters is much lower than that of the amorphous polyesters of comparable molecular weight which are commonly used in thermosetting powder compositions. This means that powder coating compositions based on semicrystalline polyesters exhibit better fluidity of the coating film in the molten state, giving final coatings free of orange-peel effects.
Furthermore, by virtue of the crystallinity of the polyesters, the powders formulated with a suitable crosslinking agent have very good storage stability. In addition, compared to amorphous polyesters, semicrystalline polyesters provide coatings having outstanding mechanical properties.
Thermosetting compositions containing semicrystalline polyesters have already formed the subject of a number of publications in the form of articles and patents. Thus, in PCT International Patent Application WO 91/14754, thermosetting powder coating compositions are described which comprise as binder a mixture of a crosslinking agent and of a polyester containing carboxyl groups, which mixture is composed of:
(a) from 5 to 100% by weight of a semicrystalline polyester containing carboxyl groups, having an acid number of from 10 to 70 mg of KOH/g and a hydroxyl number of less than 11 mg of KOH/g, and
(b) from 0 to 95% by weight of an amorphous polyester containing carboxyl groups, having an acid number of from 15 to 90 mg of KOH/g.
The crosslinking agent may be a polyepoxy compound or a bis(xcex2-hydroxyalkylamide).
According to the illustrative embodiments, these semicrystalline polyesters are prepared in one or two steps, from a major proportion of terephthalic acid or 1,4-cyclohexanedicarboxylic acid and a minor proportion of, on the one hand, adipic, succinic or 1,12-dodecane-dioc acid and, on the other hand, an aliphatic diol such as 1,6-hexanediol or 1,10-decanediol.
These semicrystalline polyesters possess a number-average molecular weight of between 1,600 and 12,000, one or more glass transition temperatures (Tg) below 55xc2x0 C., a melting point of from 50xc2x0 to 200xc2x0 C. and a melt viscosity of from 100 to 7,000 mPa.s at 200xc2x0 C. and from 4,000 to 20,000 mPa.s at 160xc2x0 C. The constituents of the amorphous polyesters are the usual constituents well known in the art, for example terephthalic acid, isophthalic acid, adipic acid, neopentyl glycol, 1,6-hexanediol, trimethylolpropane, etc. Despite the low Tg of the semicrystalline polyester, these powder compositions are storage stable and the thermall cured coatings have an improved appearance, are free of orange-peel effects and have superior mechanical properties, in particular excellent flexibility.
In PCT International Patent Application WO 94/02552, thermosetting powder coating compositions are described which comprise:
(a) a plasticizing semicrystalline polyester composed of linear aliphatic carboxylic diacids having from 4 to 22 carbon atoms, linear aliphatic diols having from 2 to 20 carbon atoms and, optionally, triols or triacids, this polyester having a melting point of from 40xc2x0 C. to 200xc2x0 C. and an acid or hydroxyl number of from 20 to 120; preferably, the polyester consists of more than 90 mol of 1,12-dodecanedioic acid per 100 mol of acids and of more than 90 mol of 1,6-hexanediol per 100 mol of polyols;
(b) a crosslinkable amorphous polyester; preferably, this polyester essentially consists of terephthalic acid and neopentyl glycol with a small amount of trimethylolpropane, and it possesses an acid or hydroxyl number of from 25 to 80;
(c) a crosslinking agent which, for polyesters containing carboxyl groups, is bis(N,N-dihydroxyethyl)adipamide or triglycidylisocyanurate.
By virtue of its plasticizing properties, the semicrystalline polyester present in the thermosetting powder composition which is described in this patent application makes it possible to obtain coatings having a better general appearance (gloss, surface evenness, absence of orange-peel effects) and better flexibility; furthermore, the storage stability of these compositions is not reduced by the presence of this semicrystalline polyester.
PCT International Patent Application WO 95/01407 relates to thermosetting powder composition for coatings comprising:
(a) an aliphatic semicrystalline polyester consisting of cyclohexanedicarboxylic acid and of diols having an even number of carbon atoms, ranging from 4 to 10; 1,4-butanediol is preferably used; this polyester possesses a melting temperature ranging from 60xc2x0 C. to 160xc2x0 C. and an acid or hydroxyl number of from 30 to 80;
(b) an aliphatic amorphous polyester consisting of cyclohexanedicarboxylic acid and a cycloaliphatic diol, for example 2,2,4,4-tetramethyl-1,3-cyclobutanediol or hydrogenated bisphenol A or alternatively 1,4-cyclohexanediol; this amorphous polyester possesses a glass transition temperature ranging from 50xc2x0 C. to 70xc2x0 C. and an acid or hydroxyl number for from 30 to 80;
(c) a crosslinking agent which, when the polyester contains carboxyl groups, may be triglycidylisocyanurate or a xcex2-hydroxyalkylamide.
The presence of the semicrystalline polyester in the thermosetting powder compositions makes it possible to obtain coatings whose impact strength is improved, while good weatherability is maintained, according to this patent application.
Semicrystalline polyesters and the thermosetting powder compositions containing them therefore possess, overall, properties which are more advantageous than those of amorphous polymers used alone. However, despite their advantageous properties, the semicrystalline polyesters used in thermosetting powder compositions give coatings whose weatherability leaves something to be desired.
This is why the Applicant has conducted research work with the objective of finding thermosetting powder compositions giving paint or varnish coatings whose quality is at least equal to that of coatings obtained from compositions containing semicrystalline polyesters of the prior art with regard to reactivity, melt fluidity, flexibility, impact strength, surface evenness, etc., but which, in addition, are capable of providing particularly weather-resistant coatings.
The surprising discovery has now been made that this objective may be achieved when, for preparing the powder coating compositions, a mixture of semicrystalline polyesters containing carboxyl groups and of amorphous polyesters containing carboxyl groups is used, the semicrystalline polyester being essentially prepared at the start from 1,4-cyclohexanediol as alcoholic constituent and from a straight-chain saturated aliphatic dicarboxylic acid as acid constituent, the amorphous polyester being essentially prepared at the start from isophthalic acid as acid constituent and from neopentyl glycol and/or 2-butyl-2-ethyl-1,3-propanediol as alcoholic constituent.
This is why, in accordance with the present invention, novel thermosetting powder compositions are provided which comprise as binder a mixture of polyesters containing carboxylic groups and of a crosslinking agent having functional groups capable of reacting with the carboxyl groups, characterized in that the polyester comprise:
(a) a semicrystalline polyester containing carboxyl groups, which comprises, with respect to the total amount of alcohols, of from 85 to 100 mol % of 1,4-cyclohexanediol and of from 0 to 15 mol % of at least one other aliphatic and/or cycloaliphatic polyol and, with respect to the total amount of acids, of from 85 to 100 mol % of a straight-chain saturated dicarboxylic aliphatic acid having from 4 to 14 carbon atoms and of from 0 to 15 mol % of at least other aliphatic and/or cycloaliphatic and/or aromatic polycarboxylic acid; and
(b) an amorphous polyester containing carboxyl groups, which comprises, with respect to the total amount of acids, of from 70 to 100 mol % of isophthalic acid and of from 0 to 30 mol % of at least one other aliphatic and/or cycloaliphatic and/or aromatic polycarboxylic acid and, with respect to the total amount of alcohols, of from 70 to 100 mol % of neopentyl glycol and/or of 2-butyl-2-ethyl-1,3-propanediol and of from 0 to 30 mol % of at least one other aliphatic and/or cycloaliphatic polyol.
The semicrystalline polyesters containing carboxyl groups present in the thermosetting compositions in accordance with the invention have an acid number of from 10 to 70 mg of KOH/g, preferably from 15 to 40 mg of KOH/g.
These semicrystalline polyesters containing carboxyl groups furthermore have the following characteristics:
a number-average molecule weight of between 1600 and 17,000, preferably between 2800 and 11,200;
a well-defined melting point of approximately from 60 to 140xc2x0 C., determined by differential scanning calorimetry (or DSC) according to the standard ASTM D 3418, with a heating rate of 20xc2x0 C./minute;
a glass transition temperature ranging from xe2x88x9250 to 50xc2x0 C. (DSC), according to the standard ASTM D 3418 with a heating rate of 30xc2x0 C./minute; and
a melt viscosity of from 5 to 10,000 mPa.s measured at 175xc2x0 C. in a cone-and-plate viscometer, (known by the name xe2x80x9cICI viscosityxe2x80x9d), according to the standard ASTM D 4287-88.
The acid constituent of the semicrystalline polyester used in the thermosetting compositions in accordance with the present invention contain from 85 to 100 mol % of a straight-chain saturated aliphatic dicarboxylic acid having from 4 to 14 carbon atoms. Examples of acids which may be used are succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, etc. These acids may be used in the form of the free acid or of their functional derivaties, in particular in the form of anhydrides. Furthermore, these acids may be used as a mixture, but they are preferably used alone. Among these acids, adipic acid is preferably used.
The acid constituent of the semicrystalline polyester may also contain from 0 to 15 mol % of one or more other aliphatic and/or cycloaliphatic and/or aromatic polycarboxylic acids or the anhydrides thereof, for example 1,4-cyclohexanedicarboxylic acid, fumaric acid, maleic acid, trimellitic acid (or the anhydride) or pyromellitic acid.
The alcohol constituent of the semicrystalline polyester used in the thermosetting compositions in accordance with the present invention contain from 85% to 100 mol % of 1,4-cyclohexanediol. The alcoholic constituent of the polyester may furthermore contain from 0 to 15 mol % of aliphatic or cycloaliphatic polyols, such as ethylene glycol, propylene glycol, 1,4-butanediol, 1,6-hexanediol, 1,4-cyclohexanedimethanol, hydrogenated bisphenol A, trimethylolpropane, ditrimethylolpropane, trimethylolethane, pentaerythritol and mixtures thereof. It is preferable to use 1,4-cyclohexanediol alone as the alcoholic constituent.
The amorphous polyesters containing carboxyl groups present in the thermosetting compositions in accordance with the invention have an acid number of from 15 to 100 and preferably of from 30 to 70 mg of KOH/g.
These amorphous polyesters containing carboxyl groups furthermore have the following characteristics:
a number-average molecular weight of between 1100 and 15,000, preferably between 1600 and 8500;
a glass transition temperature (Tg) of from 40 to 80xc2x0 C. (measured by DSC, like for the semicrystalline polyester);
a melt viscosity of from 5 to 15,000 mPa.s measured at 200xc2x0 C. in a cone-and-plate viscometer (known by the name xe2x80x9cICI viscosityxe2x80x9d) according to the standard ASTM D 4287-88.
The acid constituent of the amorphous polyester used in the thermosetting compositions in accordance with the present invention contain from 70 to 100 mol % of isophthalic acid, the remaining 0 to 30 mol % of acids consisting of one or more other aliphatic and/or cycloaliphatic and/or aromatic polycarboxylic acids, such as fumaric acid, maleic acid, terephthalic acid, phthalic acid, 1,2-, 1,3- or 1,4-cyclohexanedicarboxylic acids, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, etc., or the corresponding anhydrides thereof. The use of a polycarboxylic acid containing at least three carboxyl groups or the anhydride thereof, for example trimellitic acid (or the anydride) or pyromellitic acid, allows preparation of branched amorphous polyesters.
The alcohol constituent of the amorphous polyester used in the thermosetting compositions in accordance with the present invention contain from 70 to 100 mol % of neopentyl glycol or of 2-butyl-2-ethyl-1,3-propanediol, used alone or as a mixture. The remaining 0 to 30 mol % of alcohols consist of one or more other aliphatic and/or cycloaliphatic polyols chosen from ethylenes glycol, diethylene glycol, propylene glycol, 1,4-butanediol, 1,5-hexanediol, 1,4-cyclohexanedimethanol, 2-methyl-1,3-propanediol, hydrogenated bisphenol A and neopentyl glycol hydroxypivalate. For the preparation of branched amorphous polyesters, it is advantageous to use polyols containing three or four hydroxyl groups, such as trimethylolpropane, ditrimethylolpropane, trimethylolethane, pentaerythritol and mixtures thereof.
In the compositions in accordance with the invention, from 5 to 45, preferably from 10 to 30 parts by weight of the semicrystalline polyester containing carboxyl groups and from 55 to 95, preferably from 70 to 90 parts by weight of the amorphous polyester containing carboxylic groups, with respect to the total weight of these polyesters, are used.
An essential characteristic of the semicrystalline polyester containing carboxyl groups resides in the fact that the alcoholic constituent contains at least 85 mol % of 1,4-cyclohexanediol and that the acid constituent of the polyester includes at least 85 mol % of a straight-chain saturated aliphatic dicarboxylic acid having from 4 to 14 carbon atoms.
In fact, the surprising discovery has been made that a semicrystalline polyester having such a composition, when it is used with an amorphous polyester based on a major amount of isophthalic acid as acid constituent, and with a crosslinking agent, provides thermosetting powder compositions giving paint or varnish coatings possessing outstanding weatherability. In particular, the weatherability of the coatings obtained is markedly superior to that of the coatings obtained with the compositions prepared from the semicrystalline polyesters of the prior art which do not contain 1,4-cyclohexanediol. It is even surprising to observe that this weatherability is superior to that obtained with the commercially available amorphous polyesters containing carboxyl groups, which are used precisely because of their weatherability, such as polyesters based on isophthalic acid.
Moreover, the thermosetting powder compositions in accordance with the invention preserve all the above-mentioned advantages of the compositions containing semicrystalline polyestersxe2x80x94excellent storage stability and production of coatings with a smooth surface appearance having a high gloss and excellent mechanical properties which are preserved over time. It is obvious that in the powder paint industry, excellent weather-ability, combined with excellent mechanical properties imparted by the use of compositions in accordance with the invention are key factors of commercial importance.
The semicrystalline polyester containing carboxyl groups and the amorphous polyester containing carboxyl groups are prepared according to the conventional methods of synthesizing polyesters, using one-step or multi-step esterification. Preferably, the synthesis of the semicrystalline polyester is carried out in a single step. The amorphous polyester may be synthesized in one or two steps, depending on the monomers used.
For the preparation of the polyesters, a conventional reactor is used, this being equipped with a stirrer, an inert gas (nitrogen) inlet, a distillation column linked to a water-cooled condenser and a thermometer connected to a thermoregulator.
The esterification conditions used for the preparation of the polyesters are conventional, namely that a standard esterification catalyst derived from tin, such as dibutyltin oxide, dibutyltin dilaurate or n-butyltin trioctoate, or derived from titanium, such as tetrabutyl titanate, may be used in an amount of from 0 to 1% by weight of the reactants, and that optionally antioxidants, such as the phenolic compound IRGANOX 1010 (CIBA-GEIGY), or phosphonite-type and phosphite-type stabilizers, such as tributyl phosphite, may be added in an amount of from 0 to 1% by weight of the reactants.
The polyesterification is generally carried out at a temperature which is gradually increased from 130xc2x0 C. to approximately 180 to 250xc2x0 C., firstly under normal pressure, then under reduced pressure, these operating conditions being maintained until a polyester having the desired acid number is obtained. The degree of esterification is monitored by determining the amount of water formed in the course of the reaction and the properties of the polyester obtained, for example the acid number, the molecular weight or the viscosity.
When the polyesterification is completed, crosslinking catalysts are optionally added to the polyester still in the molten state, up to 1.5% by weight of the polyesters, for the purpose of accelerating the cross-linking of the thermosetting powder composition while it is curing.
In general, as catalyst, amine derivatives such as 2-phenylimidazoline, phosphines such as triphenylphosphine, quaternary ammonium salts such as tetrapropyl-ammonium chloride or tetrabutylammonium bromide, or phosphonium salts such as ethyltriphenylphosphonium bromide or benzyltriphenylphosphonium chloride, may be used.
The semicrystalline polyesters containing carboxyl groups and the amorphous polyesters containing carboxyl groups described hereinabove are intended to serve mainly as binders, in conjunction with crosslinking agents, in the preparation of thermosetting powder composition which can be used especially as varnishes and paints suitable for being applied using the technique of deposition by means of an elastrostatic or tribo-electric spray gun or using the technique of fluidized-bed deposition.
This is why the present invention also relates to the use of thermosetting powder compositions in accordance with the invention for the preparation of powder varnishes and paints, as well as to the powder varnishes and paints obtained using these compositions.
Finally, it also relates to a process for coating an article, preferably a metal article, which is characterized by application of a thermosetting powder composition in accordance with the invention to the said article by spray deposition using an electrostatic or tribo-electric gun or by fluidized-bed deposition followed by curing of the coating thus obtained at a temperature of from 150 to 220xc2x0 C. for a time of from approximately 5 to 35 minutes.
The crosslinking agent used for the preparation of the thermosetting powder compositions in accordance with the invention is chosen from compounds containing functional groups capable of reacting with the carboxyl groups of the polyesters. Among these compounds, the following may be used as crosslinking agents:
polyepoxy compounds solid at room temperature and containing at least two epoxy groups per molecule such as, for example, triglycidyl isocyanurate (like the one marketed under the name Araldite PT 610 by CIBA-GEIGY) or the epoxy resin Araldite PT 910 (from the same company);
xcex2-hydroxyalkylamides which contain at least one, preferably two bis (xcex2-hydroxyalkyl)amide groups, for example those mentioned in PCT (International Patent Application WO 91/14745 and satisfying the general formula: 
in which
R1 is H or an alkyl radical containing from 1 to 4 carbon atoms, and A represents an alkylene or aralkylene group containing from 2 to 20 carbon atoms, for example N,N,Nxe2x80x2, Nxe2x80x2-tetrakis (2-hydroxyethyl)adipamide (Primid XL 552 from EMS);
acrylic copolymers containing gycidyl groups obtained from glycidyl methacrylate and/or glycidyl acrylate and a (meth)acrylic monomer and, optionally, an ethylenically mono-unsaturated monomer different from glycidyl (meth)acrylate or from the (meth)acrylic monomer. An example of this type of acrylic copolymer is GMA252 marketed by ESTRON CHEMICAL, Inc.
The crosslinking agent described hereinabove is used in an amount of from 0.25 to 1.4, preferably from 0.6 to 1.05 equivalent of carboxyl groups present in the semicrystalline polyester and the amorphous polyester, per equivalent of epoxy or xcex2-hydroxyalkyl groups.
The thermosetting powder compositions in accordance with the invention may, for example, contain from 5 to 38.4 parts by weight of semicrystalline polyester, from 30 to 86.4 parts by weight of amorphous polyester and from 4 to 50 parts by weight of crosslinking agent with respect to the total weight of the polyesters and the crosslinking agent.
The thermosetting powder compositions in accordance with the invention may also contain various auxiliary substances used conventionally in the manufacture of powder paints and varnishes.
The auxiliary substances optionally added to the thermosetting compositions in accordance with the invention are, among others, ultraviolet-radiation-absorbing compounds such as Tinuvin 900 (from CIBA-GEIGY Corp.), light stabilizers based on sterically hindered amines (for example Tinuvin 144 from CIBA-GEIGY Corp.), other stabilizers such as Tinuvin 312 and 1130 (from CIBA-GEIGY), antioxidants (for example Irganox 1010 from CIBA-GEIGY) and phosphonite-type or phosphite-type stabilizers (for example Irgafos P-EPO from CIBA-GEIGY). The compositions in accordance with the invention may contain up to 10% by weight thereof with respect to the weight of the polyesters. A variety of mineral fillers and pigments may also be added to the thermosetting compositions in accordance with the invention. By way of examples of pigments and fillers, mention may be made of metal oxides such as titanium dioxide, iron oxide, zinc oxide, etc., metal hydroxides, metal powders, sulphides, sulphates, carbonates, silicates such as, for example, aluminum silicate, carbon black, talc, china clays, barytes, iron blues, lead blues, organic reds, organic maroons, etc. As auxiliary substances, mention may also be made of flow control agents such as Resiflow PV5 (from WORLEE) or Modaflow (from MONSANTO) or Acronal 4F (from BASF), plasticizers such as dicyclohexyl phthalate, triphenyl phosphate, auxiliary grinding aids, drying oils and degassing agents such as benzoin. These auxiliary substances are used in conventional amounts, it being understood that if the thermosetting compositions in accordance with the invention are used as varnishes, the addition of auxiliary substances having opacifying properties should be omitted.
In order to prepare the thermosetting powder compositions, the semicrystalline polyester, the amorphous polyester, the crosslinking agent and the various auxiliary substances used conventionally for the manufacture of powder paints and varnishes are dry-mixed, for example in a drum mixer. Next, this mixture is homogenized at a temperature lying within range from 80 to 150xc2x0 C. in an extruder, for example a single-screw Buss-Ko-Kneader extruder or a twin-screw extruder of the Prism or A.P.V. type. Next, the extrudrate is allowed to cool, is ground, optionally cryogenically by means of liquid nitrogen, and sieved in order to obtain a powder whose particle size is between 10, and 150 micrometers.
Instead of the above method, it is also possible to mix the semicrystalline polyester, the amorphous polyester, the crosslinking agent and the auxiliary substances in a solvent such as dichloromethane, to grind it in order to obtain a homogeneous suspension and then to evaporate the solvent, for example by spray drying at a temperature of approximately 50xc2x0 C., using methods known per se.
The powder paints and varnishes thus obtained are completely suitable for application on the article to be coated using the conventional techniques, that is to say using the well-known technique of fluidized-bed deposition or by application using an electrostatic or triboelectric spray gun. In the latter case, additives known to increase the ability to take a charge in triboelectric systems are added.
After having been applied to the article in question, the coatings deposited are cured by baking in an oven at a temperature of from 150 to 220xc2x0 C. for a time of from approximately 5 to 35 minutes for the purpose of completely crosslinking the coating.